Field of the Invention
The present invention relates to a metal-free, self-bearing optical cable composed of a cable core and cable cladding for use as an overhead cable in the field region of phase wires of a high-tension overhead line.
Optical cables are frequently laid in the region of high-tension overhead lines in one of two fundamentally different arrangements. In the first configuration, the optical cables are integrated in or clipped to phase or guard wires. In the second configuration, the optical cables are constructed as self-bearing structures and are laid, or strung, in the field region, in other words, between or under the phase wires of a high-tension line. The fundamental structure of a self-bearing optical cable in the second configuration and the capacitive couplings derive therefrom are shown schematically in FIG. 1. A single phase high-tension overhead line HF is shown, wherein a metal-free, self-bearing optical cable OK is strung in the field region and is held by a cross arm TR of a lattice tower GM. A guard wire referenced ES and a phase wire referenced PS are also shown.
Since the optical cable OK is composed of insulating materials, the jacket surface of the optical cable OK can be simulated by an equivalent circuit diagram formed of series resistances RL. Shunt capacitances CE extend between the optical cable OK and the guard wires ES, as well as between the optical cable OK and ground ED. Equivalent capacitances CP extend between the optical cable OK and the phase wire PS.
The illustrated equivalent circuit diagram also applies by analogy to a multi-phase high-tension line, wherein all of the phase conductors are shown united in an equivalent phase conductor.
It has been shown that the claddings of "fully dielectric cables" can be damaged or destroyed due to partial discharge events as a result of voltages appearing at the cable cladding, particularly for high-tension networks, such as those greater than or equal to 110 kV. Using the equivalent circuit diagram of FIG. 1, the following explanation can be given: the cable surface is effected by humidity and deposits, such as dirt, having the surface resistance RL, and as a result, assumes a voltage defined by the shunt capacitances CE and the effective partial capacitances CP in equilibrium when at an adequate distance from the grounded towers GM. In a worst case, the surface voltage at the optical cable can equal up to 50% of the phase voltage. Part of the resulting charging current flows off in the direction of the grounded tower GM via the surface resistance RL. The flowing current causes arc-overs and tracking currents of sufficient magnitude to break down the material of the outside cladding of the optical cable OK as a result of nonuniform resistances in the humidity film or in the contamination deposits. Thus, all transition conditions between damp and dry surfaces and/or between more or less contaminated surfaces are particularly critical.